The present invention relates to a fuel cell system which starts power generation of a fuel cell after removing impurities remaining in a fuel gas passage when the fuel cell is brought into operation, and a method of controlling the fuel cell system.
A fuel cell of solid polymer type has stacked structure of some tens to some hundreds layers of unit cells. Each unit cell includes electrically conductive separators sandwiching a Membrane Electrode Assembly (MEA), which has a hydrogen electrode and an oxygen electrode that together interpose an electrolytic membrane. When power generation is stopped, it is generally practiced in a fuel cell system having a fuel cell of this type that supply of a hydrogen gas (fuel gas) is terminated and a gas discharge valve provided at an outlet of hydrogen discharge passage is closed. When it has lapsed much time before starting power generation, it sometimes occurs that air (cathode gas) of the oxygen electrode permeates through the electrolytic membrane, staying around the hydrogen electrode in the fuel cell. Furthermore, replacement of a gas remaining in a fuel cell with an inert gas such as a nitrogen gas is sometimes conducted during termination of power generation. If this is adopted, it will be necessary to discharge impurities such as a nitrogen gas remaining in a fuel cell into an external environment when the fuel cell is brought to operation. Related techniques are disclosed in Japanese Published Patent Application 11-97047.
This patent document discloses an invention that a remaining inert gas is discharged by conducting steps, such as supplying air to a fuel cell after opening a hydrogen control valve and subsequently opening a hydrogen discharge valve. It also discloses a control method that power generation is started by closing the hydrogen discharge valve if a monitored output voltage of the fuel cell has reached no less than a threshold or a predetermined volume of hydrogen gas (anode gas) has been discharged.
However, the technique disclosed in the patent document, which introduces power generation by closing the hydrogen discharge valve when the output voltage of the fuel cell has reached the threshold, has a drawback that it is difficult to implement stable power generation. This drawback is ascribed to the fact that a fuel cell, whose rate of voltage rise is higher for example, probably has a large amount of impurities in a hydrogen gas passage when an output voltage of the fuel cell has reached a threshold in spite of insufficient discharging with a hydrogen gas. Namely, because the unconsumed impurities remain at a hydrogen electrode, it is difficult to provide stable power generation. On the other hand, when an output voltage of a fuel cell, which slowly rises, has difficulty in reaching a threshold even after completion of discharging of impurities, a fuel cell system tends to keep discharging a hydrogen gas until the output voltage has reached the threshold, which results in wasteful consumption of fuel.
A technique, in which completion of predetermined volume of a hydrogen gas triggers start of power generation, does not initiate power generation even if the output voltage of a fuel cell has risen sufficiently, which has raised a problem that it takes too much time in starting the fuel cell.